How much plant life is needed to keep a person alive in a sealed room? (idea)

While I do not wish to undermine this brilliant writeup, there is another, rather major factor to consider.

When plants die and decompose, the decomposition process uses about as much oxygen as the plant releases during its lifetime. Therefore, one would need to ensure that any plants 'on the brink' were removed* before they began to decompose.

This is why de-forestation does not directly harm our air source (don't get me wrong, it is a very bad thing though). Almost all our air is processed by single cell organisms living in the ocean and so forth.